The capacitive sensor sheet can detect a concavo-convex shape of a measuring object from changes in capacitance between a pair of electrode layers, and can be used for contact-pressure distribution sensors and sensors such as a strain gages. In general, the capacitance in a capacitive sensor is represented by the following formula (1):C=ε0εrS/d  (1)
In the above formula, C represents a capacitance, ε0 represents a dielectric constant in a free space, εr represents a relative permittivity of a dielectric layer, S represents an area of the electrode layer, and d represents a distance between electrodes.
Conventionally, as a capacitive sensor sheet used as the contact-pressure distribution sensor, for example, a sensor sheet including a dielectric layer made of an elastomer, and a pair of electrode layers (top electrode and bottom electrode) formed by containing an elastomer and conductive fillers, wherein the dielectric layer is sandwiched between the pair of electrode layers, is known (refer to Patent Literature 1).
Such a sensor sheet has a characteristic that the change in capacitance is relatively large since the dielectric layer is composed of an elastomer.
However, in the conventional capacitive sensor sheets to be used for the contact-pressure distribution sensors, a load distribution of the measuring object can be measured, but the amount of deformation by the load cannot be measured. For example, when the sensor sheet is attached to a soft article like a cushion and a load is applied to the sensor sheet, it is not possible to measure how the cushion is deformed.
Further, in the capacitive sensor sheet used as the contact-pressure distribution sensor, a displacement (elongation rate) of the dielectric layer during measurement is usually about several percentages. Therefore, even in the dielectric layer having flexibility, its elongation rate is about several percentages although it has bendability. On the other hand, in the capacitive sensor sheet to be used for measuring the amount of strain due to elastic deformation or the distribution of strain due to elastic deformation, it is not uncommon for a displacement (elongation rate) of the dielectric layer during measurement, depending on the form of usage, to exceed 100%.
Therefore, a high elongation rate is required of the dielectric layer of the capacitive sensor sheet to be used for measuring the amount of strain due to elastic deformation or the distribution of strain due to elastic deformation. Further, it is required of the electrode layer that when elongating the dielectric layer, the electrode layer can follow the elongation and electric conductivity is maintained (electric resistance does not increase).
The capacitive sensor sheet described in Patent Literature 1 cannot adequately satisfy such requirements, and it has been difficult to use the sensor as a capacitive sensor sheet for use in measuring at least one of the amount of strain due to elastic deformation and the distribution of strain due to elastic deformation. Particularly, when the electrode layer is formed using conductive fillers such as carbon black, a conductive path is easily cut off when the electrode layer is elongated with the elongation of the dielectric layer, and therefore the electrode layer cannot be used for measurement of the amount of strain due to elastic deformation and the distribution of strain due to elastic deformation.